In response to the problem of global warming, improvements in energy-saving technology have been demanded. Amongst many technical fields, power electronics technology to reduce energy loss during power conversion is of great importance. Although improvements to power electronics have been made regarding the technical performance using related art silicon (Si) semiconductors, performance improvement has reached a limit due to limitations of the material properties of silicon. Therefore, a silicon carbide (SiC) having a greater physical limit than silicon is desirable. In comparison to silicon, silicon carbide has much better physical properties, such as a band gap three times larger, a breakdown electric field strength ten times larger, and a thermal conductivity about three times higher, and is expected to be applied to power devices, high-frequency devices, high-temperature operating devices, and the like.
In order to promote commercialization of SiC devices, it is essential to establish a high-quality crystal growth technology and a high-quality epitaxial growth technology.
Although SiC has many polytypes, 4H—SiC has been mainly used to prepare a practical SiC device. As a substrate of the SiC device, a SiC single-crystal wafer that is machined from bulk crystals prepared by a sublimation method or the like is used, and typically, a SiC epitaxial film that becomes an active region of the SiC device is formed on the substrate by chemical vapor deposition (CVD). In the epitaxial film, a polytype that is different from a polytype used in the substrate is easily contaminated, and for example, in the case of using 4H—SiC in the substrate, 3C—SiC or 8H—SiC is contaminated. In order to suppress such contamination, epitaxial growth is generally performed by performing step flow growth (lateral growth of atomic steps) in a state where the SiC single-crystal substrate is tilted slightly.
<Step Bunching and Observation/Evaluation Thereof>
In the case where the SiC substrate has a size up to about 2 inches, an angle of 8° has been mainly used as the slight tilting angle (off-angle). In this off-angle, the terrace width of the wafer surface is small, and step flow growth can be easily obtained. However, as the off-angle becomes larger, fewer numbers of wafers are obtained from the SiC ingot. Because of this, in the SiC substrate of 3 inches or more, an off-angle of about 4° has been mainly used from the viewpoint of cost reduction. Since the terrace width of the wafer surface at the off-angle of about 4° is twice the terrace width at the off-angle of about 8°, variations tend to occur in the velocity of migration of atoms that are taken into the step edge, that is, in the growth rate of the step edge. As a result, a step having a high growth rate catches up with a step having a low growth rate, causing coalescence, and thus step bunching occurs. Particularly, in the case where the epitaxial surface is the Si surface, the migration of surface atoms is suppressed as compared with the C surface, and thus step bunching occurs easily. Here, step bunching means a phenomenon in which atom steps (typically 2 to 10 atom layers) are gathered, causing coalescence on the surface, and may indicate a step height of the surface itself. NPL 1 shows a typical step bunching.
In the related art, observation and evaluation of step bunching have been often performed by combination of an optical microscope, such as a differential interference microscope, and an atomic force microscope (AFM) with atomic resolution (for example, NPL 1 and NPL 2).
<Gas Etching and Supply of Raw Material Gas>
When forming a SiC epitaxial film on a SiC single-crystal substrate, in the related art, chemical-mechanical polishing (CMP) and gas etching are performed in order after performing mechanical polishing, surface treatment of the SiC single-crystal substrate is performed, and then the SiC epitaxial film is formed by a chemical vapor deposition method. The gas etching is performed mainly by using hydrogen gas at a high temperature of about 1500° C. as pretreatment for removal of damage or polishing marks (scratches) due to the polishing process and surface flattening.
The gas etching is performed with the addition of propane (C3H8) gas that is a raw material gas of the SiC epitaxial film in a hydrogen atmosphere (PTL 1, paragraph [0002] of PTL 2, and NPL 3). As shown in NPL 3, although the hydrogen gas etching is required in order to obtain a good epitaxial surface, it appears that Si droplets occur when only hydrogen is used, and with the addition of C3H8, it is considered that the occurrence of Si droplets can be suppressed.
However, if damage or polishing marks (scratches) due to polishing remain on the substrate surface after gas etching, different polytypes, dislocations, or stacking defects are introduced into the epitaxial film formed on the substrate surface thereafter. In order to avoid this, if the etching amount is increased too much through extending of the gas etching time, surface reconstruction occurs at the substrate surface, and step bunching occurs on the substrate surface before the start of epitaxial growth.
In order to suppress the occurrence of step bunching, as a method for decreasing the etching amount, a method of performing gas etching with the addition of silane (SiH4) gas, which is a raw material gas, into hydrogen gas has been proposed (PTL 2).
In any method of PTL 1 and PTL 2, although gas etching is performed with the addition of C3H8 gas or SiH4 gas, which is a raw material gas of the SiC epitaxial film, the SiC epitaxial film is deposited with the introduction of another gas directly following the gas etching, without exhausting the added gas (FIG. 2 of PTL 1, and FIG. 4 of PTL 2). That is, before the start of the SiC epitaxial film growth, the propane (C3H8) gas or the silane (SiH4) gas is already present on the surface of the SiC substrate.
As described above, in the commonly performed method at present, which is represented by PTL 1 and PTL 2, the propane (C3H8) gas and the silane (SiH4) gas, which are raw material gases, are not simultaneously supplied when the growth of the SiC epitaxial film starts.